If you want to add muscle, eat a lot of calories (lots of protein, don't be afraid of carbs either) and lift heavy. If you can do 10 reps without a real struggle, add more weight. In my opinion, 6-8 reps is where you want to be. Getting 8 should be difficult. You have to shock your body to make it stronger. After doing your heavy weights, it's OK to lighten and do reps but get your bulk on before you go for high reps.
Work the large muscle groups hard: Chest, Back, Shoulders, legs. Heavy weight with decent rest between sets. Hit each large muscle group about once per week (give at least 3 days rest before hittin the same muscle group. Recovery is important.)